1. Field of the Invention
The present invention relates to an optical scanner employing an over-field optical system, particularly to an optical scanner which has a reflector between a polygon mirror (a light deflector) and a scanned face to uniformize light quantity distribution of scanning light on the scanned face. The optical scanner of the present invention is useful for electrophotographic printers such as laser beam printers (LBP) and digital copying machines.
2. Related Background Art
The optical scanner for conventional laser beam printers or like printers conducts image recording through the steps of modulating optically a light flux (light beam) emitted from a light source in accordance with image signals; deflecting periodically by a light deflector such as a polygon mirror the optically modulated light; condensing the light into a spot on a photosensitive recording medium face with an image focusing system having prescribed fe characteristics; and scanning the face of the photosensitive recording medium with the light spot.
FIG. 1 is a schematic perspective view of the main portion of a conventional optical scanner.
In FIG. 1, a light flux emitted from light source 91 is substantially collimated by condenser lens 92. The light quantity of the light flux is limited by aperture stop 93, and the light flux is introduced into cylindrical lens 94 having a prescribed refractivity only in a sub-scanning direction. The substantially collimated light flux introduced into cylindrical lens 94 is emitted in the substantially collimated state in the main scanning plane direction, whereas in the sub-scanning direction, the light flux is focused on deflection face (hereinafter referred to as a "facet" occasionally) 96 of light deflector 95 constituted of a rotating polygon mirror to form substantially a line image.
The light flux deflected by deflection face 96 of optical deflector 95 is introduced through focusing optical system 97 having prescribed f.theta. characteristics and return mirror 98 onto photosensitive drum face 99, namely a scanned face. The light flux is allowed to scan photosensitive drum face 99 in arrow B direction (main scanning direction) by rotating optical deflector 95 in arrow A direction to record the image information.
In recent years, a higher speed is demanded for the optical scanning system. For example, an over-field optical scanning system (hereinafter referred to as OFS) is re-considered which introduces light flux having a breadth larger than the light deflector face breadth along the main scanning direction.
FIG. 2 is a schematic plan view of the main portion of an optical deflector of this kind of over-field optical scanning system.
In FIG. 2, incident light flux 82 is introduced from a light source (not shown in the drawing) in a range broader than the main scanning direction of facet 96 of polygon mirror as light deflector 95. The angle between the incident light flux and the normal line of the facet face is hereinafter represented by .theta..sub.i. A part of incident light flux 82 is deflected by facet 96 as deflected light flux 85 to be introduced onto a scanned face (not shown in the drawing). The breadth of deflected light flux 85 in the main scanning direction varies depending on the angle of deflection by reflection by polygon mirror 95. In other words, the F-number varies with the view angle. Since the intensity of the deflected light flux 85 varies in proportion to the F-number of the deflected light flux 85, the incident light flux 82 having a flat Gaussian distribution of the intensity results in a nonuniform light quantity distribution (light quantity distribution in the line image) on the scanned face (photosensitive drum face).
To solve the above problem, Japanese Patent Application Laid-Open No. 8-160338 discloses an optical scanner in which the nonuniformity of the light quantity distribution caused by the F-number variation of the deflected light flux is corrected by providing a filter having a light transmittance distribution on the optical path between the light source means and the light deflector. However, the device disclosed in the above Laid-Open publication has disadvantages that the light transmittance cannot readily be varied as desired and the tolerance is close in the range since the light flux breadth is small between the light source and the light deflector. Otherwise, with a filter having discontinuous transmittance distribution, the light quantity distribution on the scanned face cannot smoothly be corrected, tending to cause streaks in the output image, disadvantageously.
In other examples, the light quantity distribution on the scanned face is corrected to be substantially uniform in an under-field optical system in which the light flux having a breadth smaller than that of the facet is introduced to a light deflector. For example, Japanese Patent Application Laid-Open No. 9-80334 discloses an optical scanner in which the variation of the light quantity with light emission time length is corrected by varying the reflectivity of a return mirror to uniformize substantially the light quantity distribution on the scanned face.
In an example of practical optical scanners, the drop of the light quantity at the peripheral portion by variation of the transmittance of the focusing optical system is corrected by varying the reflectivities of two return mirrors to uniformize substantially the light quantity distribution on the scanned face.
In the above examples, however, no consideration is taken on the drop of the light quantity caused by the variation of F-number of the deflected light flux and the gradient in Gaussian distribution of the light intensity which are inherent to the OFS system. In the OFS system, the nonuniformity of the light quantity distribution on the scanning face is caused by the drop of the light quantity by variation of F-number of the deflected light flux and the gradient in the Gaussian distribution of the intensity much more than by the time length of light emission and the variation of the light transmittance of the focusing optical system.
The present invention is made in comprehensive consideration of the drop of the light quantity at the peripheral portion by the variation of F-number of the deflected light flux and the gradient in Gaussian distribution of the light intensity in the OFS system.